1. Field of the Invention
The present invention relates to a control unit and a display unit for an EMG controlled ventilator.
2. Description of the Prior Art
U.S. Pat. No. 5,671,752 discloses a method and a device for registering the myoelectrical activity of the diaphragm by means of an esophageal catheter having an array of electrodes. Such a signal from an esophageal catheter is prone to disturbances from other myoelectrical signals that are present in the patient's body. For improving the signal-to-noise ratio of such an electromyographic signal cross-correlation of the signals from the different electrodes in the catheter is used. Electrode pairs on opposite sides of the diaphragm but having approximately the same distance to the diaphragm will produce signals that are opposite but substantially equal in magnitude and phase. By subtracting the signals from one electrode pair from the other the two desired signals will be added, while the noise components of the two signals will substantially cancel each other out.
When a patient is breathing spontaneously but still needs breathing support the myoelectric signal from the diaphragm can be used to control the ventilator. U.S. Pat. Nos. 5,820,560 and 6,588,423 both disclose methods and devices for triggering ventilatory support to a patient using a myoelectrical signal obtained from the diaphragm. WO05/048838 discloses how to obtain an EMG signal from an esophageal catheter, taking into account the ECG signal, and in particular handling the fact that the position of the esophageal catheter relative to the diaphragm may change rapidly.
A problem when obtaining a myoelectrical signal from the diaphragm is positioning of the catheter within the patient's esophagus. To obtain a proper signal some of the electrodes should be placed above the diaphragm and some below it. There is a possibility that the catheter will be inserted too far, or not be inserted far enough. In both cases the catheter will detect a weak signal or may not capture any signal at all. The catheter may also capture myoelectrical signals from other muscles instead of, or in addition to, the signal from the diaphragm. Hence, it is difficult to obtain an optimal catheter position and the ventilator may have to work in pneumatic triggering mode if the signal is too weak.
Ensuring the correct positioning of the catheter within the patient is therefore important.
Also, the diaphragm will move up and down as the patient breathes, so that the position of the electrodes relative to the diaphragm will change to a degree that is dependent on the breathing activity of the patient. Hence, even if the esophageal catheter is placed correctly initially, it may move in such a way that it no longer records the EMG signal of the diaphragm.
There is also a risk that the catheter will be moved inadvertently, to be inserted too far into the patient or be pulled out from the patient's esophagus. Therefore, the position of the catheter should be monitored continuously to ensure that a correct signal is obtained and the operator should be notified of any changes.